The mechanism by which mammalian cells regulate their proliferative capacity are not known but events occurring in the G1 phase of the cell cycle are thought to be of major importance. In order to facilitate the study of cell-cycle events important to cell growth control, we have cloned mouse DNA segments containing sequences whose expression is induced by mitogenic stimulation of cultured mouse embryo cells by epidermal growth factor (EGF). Twelve clones have been derived. Nine were found to be complementary to RNA sequences exhibiting a 3-\to 7-fold increase in mass per cell by 6 hours following EGF stimulation. Three proved to contain sequences complementary to RNA species exhibiting a 20-\to 80-fold increase following EGF stimulation. In addition to these mouse sequences, nine clones have been selected from a human gene library on the basis of sequence conservation with EGF-inducible mouse mRNAs. This research utilizes these and other similarly derived clones to study G1-specific events which occur following the stimulationof quiescent cells to re-enter the cell cycle. Specifically, we will identify mRNA sequence corresponding to the cloned DNAs and, where possible, identify the translation products. Experiments will elucidate the patterns of expression of these clones following EGF stimulation, their responsiveness to other mitogens, and the mechanisms whereby the individual genes are regulated. These experiments will provide new knowledge concerning the regulation of cell proliferation which may ultimately prove relevant to the understanding of both cell transformation and cell senescence.